Steel responsive to austempering



United States Patent STEEL RESPONSIVE TO AUSTEMPERING Leonard V. Klaybor, Dunkirk, N. Y., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Application August 23, 1956 Serial No. 605,720

3 Claims. (Cl. 75-128) This invention relates to improvements in steels and in particular to steels which are capable of substantially uniform response to an austempering treatment.

Steels used for lawn mower blades, chain saw cutting bits, impact knives and other related applications are subjected to severe impact shock, high abrasive wear and flexing. Thus such steel must possess a high hardness to adequately perform the intended task. To meet these severe requisites of service, steels of the medium'carbon content to which slight amounts of alloying elements were added have been employed in these applications, said steels having a hardness of between 50 and 53 R Steels of such hardness appear to have the best balance of the required physical properties. However, in achieving this balance of physical properties, adequate ductility and impact strength were obtained by sacrificing hardness and resistance to abrasion. Since the ultimate of all these physical properties would result in better service performance it is apparent that, if ductility and impact strength could be achieved without the sacrifice of hardness and wear resistance, a great improvement would result in the service life of articles fabricated from such steels.

In order to alleviate this condition, it has been found that through a critical balancing of small amounts of the alloying elements together with the heat treatment known in the trade as austempering, the desired optimum balance of the aforementioned physical properties could be achieved. Present steels in use, however, are deficient from the standpoint of being nonuniform in their response to the austempering treatment. Slight differences of temperature and time, which are always a normal part of production heat treatment, are 'suflicient in themselves to give nonuniform hardnesses, ductilities and impact strengths. .Even by employing the quench and temper heat treatment substantially nonuniform results followed.

An object of this invention is to provide a steel having the characteristics of being resistant to severe impact shock, abrasive wear and flexing.

Another object of this invention is to provide a steel having a .high carbon content with substantially. small amounts of alloying elements and which will substantially uniformly respond to an austempering heat treatment.

A more specific object of this invention is to provide a steel comprising carbon, manganese, silicon, chromium, nickel and vanadium and which will substantially uniformly respond to an austempering treatment to provide a hardness of between about 53 R and 56 R and provide an optimum balance between hardness, wear resistance, ductility and impact strength.

Other objects of this invention will become apparent when read in conjunction with the following description.

In its broader sense, this invention contemplates a steel containing between about 0.75% and about 1.0% carbon, up to about 0.75% manganese, up to about 0.50% silicon, from about 0.10% to about 1.0% chromium, from about 0.10% to about 1.0% nickel, from about 0.10% to about 0.50% vanadium and the balance substantially iron with incidental impurities.

Each of the alloying elements act singly or in combination with each other to give high wear resistance, excellent ductility and impact strength at high hardness levels after an austempering heat treatment. Thus the vanadium and some of the carbon combine to form vanadium carbide which improves the abrasion resistance of the steel. Since vanadium carbide does not dissolve during the high temperature heat treatment, it acts as an inhibitor to grain growth through a wider range of temperature and time cycles at the austeriitizing temperature thereby giving a more uniform response during the austempering treatment. Manganese in amounts up to 0.75% and nickel up to 1.0% improve the hardenability of the steel and at the same time contribute to the toughness of the matrix of the steel because of their solubility in iron within the limits of the compositional range as set forth hereinafter. Chromium in the steel of this invention together with a portion of the iron forms iron chromium carbides which are readily soluble at the austenitizing temperature, thus increasing the hardenability of the steel. Upon austempering the ironchromium carbides precipitate submicroscopically thereby contributing to the abrasion resistance of the hardened steel. A carbon content lower than 0.75% does not give the-required abrasion resistance nor hardness necessary for the intended use of this steel whereas a carbon content inexcess of 1.0% appears to reduce the impact strength and ductility of the resulting steel. Manganese and nickel contents higher than the ranges given therefor make the steel difiicult to anneal and to blank or form in the annealed state. A vanadium content in excess. of 0.50% produces an excess of vanadium carbide within the microstructure which has the effect of reducing the impact strength while increasing the wear resistance. Vanadium contents of less than 0.10%, however, do not give sufficient improvement in wear resistance. Further, it has been found that at least 0.10% vanadium is necessary in order to obtain a substantially uniform response to the austempering heat treatment to be described hereinafter.

Referring to Table I there are set forth the chemical composition of the general range of the steel of this invention, the optimum range and two steels identified as 482D and 25385H which are within the limits of the optimum range of the steel of this invention.

Table .1

[Chemical composition-Percent by Wt.]

General Optimum 482D 25385H Range Range 0.80-0.90 0.83 0.85 0. 20-0. 40 0.21 0. 28 0. 100. 30 0.12 0. 25 0. 18-0. 35 0. 19 0.25 0. 40-0. 70 0. 61 0. 54 0. 200. 25 0. 21 0. 24 Bal. Bal. Bal.

The steel of this invention can be made by conventional steel making practices and is handled in every respect as a normal steel mill product. Since the steps of making and handling the steel are well known in the art they will not be described in detail. It is sufficient to point out that the steel may be forged, rolled, hot shaped or hot formed at a temperature in the range between 1600 F. and 2000 F. The steel may be annealed after hot Working or after it has been hardened by heating to a temperature in the range between 1450 F. and 1500 F., holding the steel at this temperature for a suitable period of time and thereafter slowly cooling the steel to room temperature. When annealed in this manner, the steel has a hardness of between about 160 BHN and 196 BHN. The steel may thereafter be hardened by heating to a temperature in the range between 1525 F. and 1575' F. followed by an oil quench to room temperature. Typical hardness values of between 62 R and 64 R are obtained by such treatment.

It is preferred to heat treat the steel of this invention by austempering in order to obtain the optimum balance between physical properties and provide for a substantially uniform response to heat treatment. The austempering heat treatment can be conveniently accomplished by heating the steel to a temperature between 1525 F. and 1575 F. for a period of time sufficient to produce a substantially uniform temperature within the steel being heat treated. From this temperature the steel is quenched in an isothermal transformation bath at a temperature in the range between 500 F. and 600 F. Holding the steel at this temperature range gives a hardness of between 53 R and 56 R This heat treatment results in the transformation of austenite to bainite as a hardening microstnlctural constituent thereby giving increased impact strength and ductility. Extended holdings at the temperature between 500 F. and 600 F. point up the nonsensitivity of this steel to the time period in the austempering bath. For example, by quenching the steel from the austenitizing temperature of 1550 F. to the austempering temperature of about 550 F. and holding at this temperature for approximately 10 minutes the steel has a hardness of about 55 R If the time period in the austempering bath is increased .to 60 minutes at the same temperature of 550 F., the hardness of the steel is still 54 R The slight decrease in hardness over a wide range of time points up the nonsensitivity of the steel to time in the austempering bath.

In order to more clearly demonstrate the substantially uniform response to the austempering heat treatment, reference may be had to Table II which shows the .efiect of the time cycle on the hardness and bend angle ductility of samples taken from Heat 482D identified hereinbefore. Such samples measured /2 inch x 3 inches X .080 inch and were austenitized at 1550 F. for 10 minutes prior to immersion in the austempering bath.

TABLE II Salt Temp., F. Time, Results Austompered Mins.

Hardness, Rt

55. 5 Brittle.

Ductile. Do. Do.

Do. Do.

4 minutes the alloy had a bend angle ductility which is recorded as ductile. In the bend test the samples termed as ductile were deformed 60 from the vertical, returned to the vertical and then 60 in the opposite direction without breaking. It will, of course, be appreciated that where the thickness of the steel is less than 0.080 inches less time in the austempering bath may produce a ductile material. Conversely, as is well known in the art, larger sections than those described here may require larger times than 10 minutes to produce ductile material.

Production tests on chain saw cutting bits made from the steel of Heat No. 25385H indicated that the wear resistance was substantially improved over the wear resistance of known steels and that the ductilities as measured by bend testing at a hardness level of 55 R showed improvement of about 50% in the bend angle ductility over presently used materials. It has also been found that the forming and blanking operations of the chain saw cutting bits from the steel in the annealed conditions showed a substantially marked improvement in the blanking and forming properties over and above those of existing steels.

Since there are no special skills nor equipment needed to produce the steels of this invention, it is readily adaptable to present steel making operations. The austempering heat treatment is well known in the art and is also adaptable .to large scale production heat treatment of the .steel of this invention.

I claim:

1. A steel consisting of, from 0.80% to 0.90% carbon, from 0.20% to 0.40% manganese, from 0.10% to 0.30% silicon, from 0.18% to 0.35% chromium, from 0.40% to 0.70% nickel, from 0.20% to 0.25% vanadium, and the balance iron with incidental impurities, said steel being characterized by being responsive substantially uniformly to austempering treatment to develop a hardness of at least 53 R 2. A steel consisting of about 0.83% carbon, about 0.21% manganese, about 0.12% silicon, about 0.19% chromium, about 0.61% nickel, about 0.21% vanadium, and the balance iron with incidental impurities, said steel being characterized by being responsive substantially uniformly to austempering treatment to develop a hardness of at least 53 R 3. A steel consisting of about 0.85% carbon, about 0.28% manganese, about 0.25% silicon, about 0.25 chromium, about 0.54% nickel, about 0.24% vanadium, and the balance iron with incidental impurities, said steel being characterized by being responsive substantially uniformly to austempering treatment to develop a hardness of at least 53 R References Cited in the file of this patent UNITED STATES PATENTS 868,327 Churchward Oct. 15, 1907 FOREIGN PATENTS 344,822 Great Britain Mar. 11, 1931 

1. A STEEL CONSISTING OF, FROM 0.08% TO 90% CARBON, FROM 0.20% TO 0.40% MANGANESE, FROM 0.010% TO 0.03% SILICON, FROM 0.18% TO 0.35% CHROMIUM, FROM 0.40% TO 7.70% NICKEL, FROM 0.20% TO 0.25% VANDADIUM AND THE BALANCE IRON WHICH INCIDENTAL IMPURITIES, SAID STEEL BEING CHARACTERIZED BY BEING RESPONSIVE SUBSTANTIALLY UNIFORMLY TO AUSTEMPERING TREATMENT TO DEVELOP A HARDNESS OF AT LEAST 53 RC. 